The efficient, commercial production of biofuels from plant material, such as sugarcane, requires the fermentation of pentoses, such as xylose. Xylose in plant material typically comes from lignocellulose, which is a matrix composed of cellulose, hemicelluloses, and lignin. Lignocellulose is broken down either by acid hydrolysis or enzymatic reaction, yielding xylose in addition to other monosaccharides, such as glucose. (Maki et al., 2009, Int. J. Biol. Sci. 5:500-516).
Fungi, especially Saccharomyces cerevisiae, are commercially relevant microorganisms that ferment sugars into biofuels such as ethanol. However, S. cerevisiae does not endogenously metabolize xylose, requiring genetic modifications that allow it to convert xylose into xylulose. Other organisms, whose usefulness in ethanol production is limited, are able to metabolize xylose (Nevigot, 2008, Micobiol. Mol. Biol. Rev. 72:379-412).
Two pathways have been identified for the metabolism of xylose to xylulose in microorganisms: the xylose reductase (XR, EC 1.1.1.307)/xylitol dehydrogenase (XDH, EC 1.1.1.9, 1.1.1.10 and 1.1.1.B19) pathway and the xylose isomerase (XI, EC 5.3.1.5) pathway. Use of the XRIXDH pathway for xylose metabolism creates an imbalance of cofactors (excess NADH and NADP+) limiting the potential output of this pathway for the production of ethanol. The XI pathway, on the otherhand, converts xylose to xylulose in a single step and does not create a cofactor imbalance (Young et al., 2010, Biotechnol. Biofuels 3:24-36).
Because S. cerevisiae does not possess a native XI, it has been desirable to search for an XI in another organism to insert into S. cerevisiae for the purpose of biofuels production. Several XI genes have been discovered, although little or no enzymatic activity upon expression in S. cerevisiae has been a common problem. The XI from Piromyces sp. E2 was the first heterologously expressed XI in S. cerevisiae whose enzymatic activity could be observed (WO 03/062430).